In its broadest sense, a filter encompasses any electrical network capable of selecting signals within a specified frequency range. One type of filter is commonly known as a "band-pass" filter. For such a filter, signals within a specified frequency band pass through the filter unattenuated while signals outside the specified frequency band are greatly attenuated. Like other types of filters, a band-pass filter may be configured to include only passive components such as resistors, capacitors and inductors or may alternately be configured to include active components such as transistors and integrated circuits. A specific class of filters, commonly referred to as "high Q-factor" or "high Q" filters, are particularly favored for narrow band-pass applications. The Q of a filter is the ratio of the reactance to the resistance of the electrical network which forms the filter and "high Q" filters refer to those filters in which the aforementioned ratio is on the order of about 1,000. High Q filters typically provide a response characteristic having very narrow pass bands, i.e., a response characteristic having on the order of about a 3 dB drop for a 5% shift off its resonant pass frequency.
A commonly recognized drawback to existing filter designs is the existence of a time delay between application of a signal to an input side of a filter in a rest state and entry of the filter into an active state whereby the filter's response characteristic is applied to the input signal. While the aforementioned time delay, which will hereafter be referred to as the "response delay" of a filter, will vary depending on the particular design of the filter, a typical response delay for a filter may range between 100 .mu.sec and 2 msec. As a result, therefore, filters often fail to detect input signals of relatively short duration, commonly known as "bursts", particularly when the duration of the burst is less than the response time of the filter. While response delays occur in all conventional filter designs, they are of particular concern for high Q, narrow band-pass filters which typically include more and/or larger components which require longer charging times. Thus, it has been generally recognized that, as the band-pass for a filter is narrowed and its Q is increased, the response delay for that filter increases. Thus, the design of a filter capable of passing an increasingly narrow frequency band while maintaining an acceptable response delay characteristic has remained problematic.
Turning to FIG. 1a, a response characteristic 10 for a conventional narrow band-pass filter (not shown) may now be seen. As may now be seen, the narrow band-pass filter is tuned to frequency f.sub.C. As the input signal sweeps through a range of frequencies, the amplitude of the response characteristic will start at zero, gradually increase, peak at center frequency f.sub.C and then gradually decrease back to zero. FIG. 1b illustrates the response delay which occurs in the narrow band-pass filter having the response characteristic illustrated in FIG. 1a in response to the application of a signal having frequency f.sub.C to an input side thereof. At time t.sub.0, both the input signal 12 applied to the narrow band-pass filter and the output signal 14 produced by the response characteristic of the narrow band-pass filter are at zero. At time t.sub.2, a burst which lasts until time t.sub.4, is applied to the input side of the narrow band-pass filter. If the narrow band-pass filter had an ideal, i.e., 0 sec., response delay, the output signal 14 resulting from applying the response characteristic 10 to the input signal 12, would also appear at time t.sub.1. However, the narrow band-pass filter will remain inactive for a latency period 16 which extends from the application of the input signal at time t.sub.1 to time t.sub.3. During the latency period 16, the narrow band-pass filter is storing current in a start-up resonant mode. From time t.sub.1 to time t.sub.2, no output signal passes through the narrow band-pass filter. As the burst continues past time t.sub.2, energy begins to build in the form of a resonance. Accordingly, an output signal begins to build at the output of the narrow band-pass filter. When the level of the output signal reaches the average signal level of the input signal at time t.sub.3, the latency period 16 ends and a stimulation period 18 begins.
A similar delay in the response of the narrow band-pass filter occurs upon removal of the input signal. 12. At time t.sub.4, the input signal 12 applied to the input side of the narrow band-pass filter is removed. Upon removal of the input signal 12, the output signal 14 will continue at full strength for a short time period, i.e., until time t.sub.5, and then begin to taper off to until returning to zero at time t.sub.6.
It should be readily understood that there would be innumerable advantages for implementing a "real-time" narrow band-pass filter, i.e., a narrow band-pass filter having a response characteristic enhanced by the reduction or elimination of the response delay. Such a filter would have innumerable applications in industry. For example, as a filter having little or no response delay would be able to extract a desired signal from a relatively short signal burst, resolution of the desired signal on a cycle-by-cycle basis becomes possible. Currently, however, no real-time implementation of a narrow band-pass filter characterized by little or no response delay are known. It is noted that several non-real-time solutions to the problems caused by the response delay of filters have been proposed. Such solutions generally involve sampling the input signal and performing a mathematical analysis of the data in the frequency domain. While such techniques can resolve very short signal bursts occurring at a specific frequency, the computational time required can be rather significant. Thus, non-real-time solutions are not particularly suited for most filter applications.
What is needed, therefore, is a band-pass filter having a narrow pass response characteristic similar to existing high Q band-pass filters but avoids the response delay of such filters.